Just to confirm what Brian is saying - I had an almost identical problem with my 5 Port some time ago. Piston and head really black and blowing plugs all the time. It was exactly as Brian suggests. The top end was too lean and the bottom was way too rich (about 1.5 turns in my case) and yet the motor ran OK but did get hot at the end of a tank, hence the wear on the plugs.
Cured mine by setting HSN flush and LSN 1.5 in from flush, then tuning the LSN leaner 1/8th of a turn at a time until I got a steady idle that didn't drop and good take off. Then fine tuned with pinch test. Just out of interest you may want to check the idle gap as well. The other part of my problem was the idle gap was too wide, giving a false tune and idle that seemed OK but in fact the rich bottom end was compensating for a too wide an idle gap and sort of ¨flooding¨ the motor into running what seemed like normal. Set idle gap at .7 mm and tune from there. When both needles are set correctly then adjust the idle gap if necessary to get the idle speed you want.